Method for production of extruded, polystyrene-based resin foam plate and extruded foam plate obtained thereby

A method for the production of an extruded, polystyrene-based resin foam plate comprising the steps of heating and kneading a polystyrene-based resin together with a blowing agent, a flame retardant and a nucleating agent in an extruder to obtain a foamable molten composition, and extruding said foamable molten composition through a die attached to an end of said extruder into a lower pressure zone, wherein said blowing agent comprises:

• (a) not less than 25% by weight but not greater than 65% by weight, based on the total weight of said blowing agent, of an isobutane-containing saturated hydrocarbon;
• (b) not less than 5% by weight but not greater than 70% by weight, based on the total weight of said blowing agent, of a dimethyl ether-containing ether;
• (c) not less than 5% by weight but not greater than 55% by weight, based on the total weight of said blowing agent, of carbon dioxide;
• (d) 0–25% by weight, based on the total weight of said blowing agent, of other blowing agent components;
• wherein said isobutane is at least 46.1% by weight of said blowing agent;
• and
• wherein said foam plate has a thermal conductivity of not greater than 0.030 W/m·K and contains residual isobutane in an amount of 0.45–0.80 mol, per 1 kg of said foam plate.